In modern semiconductor fabrication facilities, overhead shuttle (OHS) systems and overhead hoist transport (OHT) systems are extensively used to automate the wafer transport process. The OHS is a transport mechanism suitable for long distance interbay transport and the OHT is suitable for short distance intrabay transport. Based on dispatching need and by the instruction of an automated material handling system (AMHS), a stocker can store wafer carriers for the OHS and the OHT before the wafer carriers are further transported. A wafer carrier may move within the same transport system, or the wafer carrier may be routed from the OHS to the OHT or vice versa. For cross-system transportation between the OHS and the OHT, a robot inside the stocker is configured to transfer the wafer carrier from one transport system to the other.
Although the robot inside the stocker performs cross-system transportation between the OHS and the OHT, the robot is also used to transfer wafer carriers between the stocker and either the OHS or the OHT if the movement of wafer carriers is within the same transport system. The robot inside the stocker serves both the needs of single-system transportation and cross-system transportation. Therefore, the robot may be overloaded and cause traffic congestion in times of heavy traffic. In addition, regarding space utilization, the stocker will occupy the shop floor of a semiconductor manufacturing facility for storage of wafer carriers. The shop floor in a clean room of the semiconductor manufacturing facility is valuable and limited.
The dual roles of the robot to facilitate the storage function of a stocker for the same transport system and to perform cross-system transportation make the robot a potential bottleneck of the overall transportation of wafer carriers. The stocker and the robot might not meet the expectation of lean operation in semiconductor fabrication facilities. In addition, if the stocker is used solely for cross-system transportation, the shop floor occupied by the stocker would be less cost effective.